In long range and secure radio frequency communications systems it is often desirable to be able to detect a signal which is at or below a noise level. In secure communications systems in particular, it may be necessary to process a transmitted signal so as to spread it into the noise background and to reconstitute the signal at a receiver by applying an autocorrelation process. Surface acoustic wave (SAW) matched filters are widely used to perform these functions.
A particular type of SAW filter useful for these purposes is a biphase matched filter. The signal input to a biphase filter is spread in time and is periodically phase inverted according to a code determined by the SAW transducers. The degree of security of a biphase processed signal is determined by the number of bits in the code, which corresponds to the number of electrode pairs in the biphase transducer. However, very large numbers of electrode pairs create multiple reflections and regeneration of the surface acoustic wave signal which degrade device performance, thus limiting the degree of security obtainable with acceptable levels of spurious signals.